Tree biodiversity of warm drylands is likely to decline in a drier world.

Warm drylands represent 19% of land surfaces worldwide and host ca. 1100 tree species. The risk of decline due to climate aridification of this neglected biodiversity has been overlooked despite its ecological and societal importance. To fill this gap, we assessed the risk of decline due to climate aridification of tree species in warm drylands based on spatialized occurrence data and climate models. We considered both species vulnerability and exposure, compared the risk of tree species decline across five bioregions and searched for phylogenetic correlates. Depending on the future climate model, from 44% to 88% of warm drylands' tree species will undergo climate aridification with a high risk of decline even under the most optimistic conditions. On a regional scale, the rate of species that will undergo climate aridification in the future varies from 21% in the Old World North, to 90% in Australia, with a risk of decline confirming the high level of risk predicted at the global scale. Using generalized linear mixed models, we found that, species more exposed to climate aridification will be more at risk, but also that species vulnerability is a key driver of their risk of decline. Indeed, the warm drylands specialist species will be less at risk due to climate aridification than species being marginal in warm drylands. We also found that the risk of decline is widespread across the main clades of the phylogeny and involves several evolutionary distinct species. Estimating a high risk of decline for numerous tree species in all warm drylands, including emblematic dryland endemics, our work warns that future increase in aridity could result in an extensive erosion of tree biodiversity in these ecosystems.